Finn's sister
by terminatorluvr
Summary: Finn's sister Fionna has just finished her training to becoming a hero like her twin brother but how will Jake and the others react when she arrives with her boyfriend Marshall Lee, her best friend Cake and a parade of other characters
1. Chapter 1

Jake was sitting on the couch with BMO as they ate a packet of cheese puffs together not really caring about the rest of the world. Suddenly Finn burst in with a bunch of decorations and food which made Jake excited. "Hey Finn are we having a party?" Jake asked happily leaping up and down.

"BMO provide music" BMO cried cheerfully and began playing a tune.

"Not now man I'm busy preparing for my sister's arrival she's coming back to Ooo after her studying to become a heroine in another kingdom" Finn said sternly.

Jake looked stunned "I didn't know you had a sister what's her name?" he asked curiously.

"Her names Fionna and she has a best friend named Cake she's a talking cat their coming back to live with us" Finn said as he rushed around hanging up decorations.

Jake almost fainted "But Finn this is our place bro no girl remember" he said his tail drooping.

Finn patted his shoulder "I know dude but she's not a girly girl she's a tomboy and she really is a fun person plus she's inviting all her friends" he reassured him.

Jake perked up a little "Ok I'll go call our buddies" he said making his way to the phone.

_**On phone**_

(In Korean) "Hello who is this?" Lady Rainicorn asked politely.

"Hey lady it's me Jake we're having a party to celebrate the homecoming of Finn's sister and her new title of becoming a Heroine" Jake said lovingly down the phone.

(In Korean) "Oh Jake how nice to hear from you I'd love to come" Lady Rainicorn said she continued to talk flirtatiously with him before putting down the phone. He then began to call others and before long he had gone through their entire list of friends.

_**After phone call**_

Jake made his way outside where Finn was setting up tables for the food and putting decorative lights all around the tree house. "Hey Jake so what did Lady Rainicorn say" Finn asked as he put the finishing touches.

"She's gonna come tonight as are PB, Marceline and some of the candy people" Jake called up to him.

"Great we'll have more people to mingle with especially since it's my sister she's got heaps of friends I'm sure it'll be a blast" Finn said sorting the last decoration.

_**Later on**_

"Mmm great sandwiches Jake what filling is it?" Princess bubblegum asked as she ate her fill happily.

"Oh meat, cheese, salad, pickles all sorts really" Jake said as he ate his own sandwich pleasantly.

Lady Rainicorn kissed Jake "You'd make a better snack than these nibbles (In Korean)" she said lovingly; She fluttered her eyelashes in a flirtatious manner towards Jake and giggled.

Jake blushed and wriggled about "Aaaw lady c'mon" he said shyly.

"So you're the friend of Fionna's brother are you?" a voice said from behind him and he saw a very cute looking orange and white cat behind him.

"Yeah and what of it?" he said suspiciously pacing his hands on his hips.

"My name is Cake I'm the roommate and best friend of Finn's sister Fionna" the cat said shaking Jake's hand.

Lady Rainicorn spotted a black unicorn over by the punchbowl and decided to say hello to the black unicorn/horse by the punch bowl.

"Hello my name is Lady Rainicorn and I'm Jake's girlfriend (in Korean)" she said to Lord Monochromicorn.

Lord Monochromicorn looked at her then stomped back in Morse code "Nice to meet you I'm Cake's boyfriend Lord Monochromicorn".

"They seem to be getting along well" Princess bubblegum said to peppermint butler as she poured herself some punch.

"C'mon Fionna lets go somewhere just the two of us we're a couple now right" Marshall Lee groaned crossly his face irritated. He didn't like sharing Fionna with other people not now that she'd agreed to be his girlfriend.

"C'mon Marshall you agreed to behave tonight I haven't seen my brother for ages" Fionna said lovingly and kissed his cheek as she led him along. She spotted Finn over by the punchbowl speaking to the vampire queen Marceline.

"Hey Finn how are you big bro?" she asked happily rushing up to Finn and hugging him tightly but everyone was shocked when they saw how alike they looked.

"Uh huh they like totally look the same its weird yet the tension is totally awesome!" Lumpy space princess said.

"Um babe are you gonna introduce me or not?" Marshall Lee asked approaching them and glaring a little at Finn.

"Oh yeah, Finn this is my boyfriend Marshall Lee the vampire king isn't he gorgeous" Fionna said nuzzling into his chest.

"But…But you look exactly the same what could you possibly have that's different besides your gender?" Princess Bubblegum cried.

"How about their clothes!" cake said crossly defending her friend.

"But they look like totally similar except the styles" Lumpy space princess added yet again.

Fionna pulled out her sword "Our swords are different, Finn has a busted up yellow one and I have a red crystal blade" she said showing off which made Finn jealous.

"Sorry dude but I gotta admit even for a girl that sword is badass" Jake said munching on crisps.

"What about their hair I wanna see their hair" Wild berry princess cried excitedly in the background jumping up and down.

Finn sighed "Fine but only one it gets messy" he said and pulled off his hat showing lovely blonde locks that fell to his shoulders in radiant waves. He then put it back under his white teddy hat afterward.

Cinnamon bun nodded "Ok now you Fionna" he said turning to the pretty young girl with a good looking sword.

Fionna looked nervous "Ok but it tends to get in the way" she said and pulled off her cute white bunny hat releasing long blonde waves that fell to her waist.

"Man she's hot" Lumpy space princess said gazing in awe at this radiant beauty. After allowing everyone to look at her hair Fionna placed her cute bunny hat back on.

"Hey I like just had a totally great idea!" Lumpy space princess cried slapping her cheeks with her hands.

"What is it LSP?" Jake asked curiously his eyes shining with excitement.

"Like Finn and Fionna could like totally protect Ooo together and keep it like safe from the Ice king" Lumpy space princess cried to which everyone agreed.


	2. C2: Living together

Finn was asleep in his red Pyjama's and dreaming of him and PB being on a date together. He was also having a mixed dream where he and Jake were eating all the pancakes they wanted at the breakfast kingdom.

"Jake…pancakes…syrup…princess bubble-gum…must kiss" Finn mumbled sleepily and tossed and turned inside his sleeping bag.

Jake was asleep in his usual drawer and dreaming of meat and lady rainicorn to which he was drooling and giggling in his sleep.

Meanwhile in the separate part of the tree house where Fionna, cake and Marshall Lee were the sleeping arrangements had been quite different and even Finn had stated his sister was bolder than most girls.

Cake was asleep in a drawer like Jake but was purring loudly and wrapped in a blue blanket her tail swishing back and forth happily as she slept.

Fionna was wearing a white blouse and panties while Marshall Lee wore a dark shirt and shorts which made him look really sexy. The two were cuddled up together in bed enjoying each other's company.

Fionna had tied her hair into a plait with a pink ribbon and was asleep curled up next to Marshall Lee. Her breasts were pressed against his chest and he wasn't bothered Marshall was enjoying this.

BMO was watching Fionna and Marshall from the bedroom floor innocently not able to understand the chemistry of human beings "I no understand? Why Finn's sister sleep with evil vampire?" he thought.

Marshall stirred and spotted the small computer sitting on the floor staring at them and frowned "Yo little dude I'm trying to enjoy some alone time with my babe".

BMO became sad and left the room to go and sleep with Finn and Jake. He wanted to get to know Cake and Fionna better but with Marshall Lee there he had no chance.

_**Morning**_

Finn and Jake were at the breakfast table with BMO and then Cake appeared and sat next to Jake while Marshall Lee and Fionna sat together.

"Um Fionna did you say anything mean to BMO last night he's really upset at you guys" Jake said curiously.

"I told the sneaky peeper to leave when he came into our room last night" Marshall confessed innocently with a sly attitude.

"Yo sis, Jake listen up LSP is throwing a party today and she's invited us you want to bring Cake and Marshall Lee" Finn asked cheerfully.  
Jake perked up from eating his giant sandwich "Hell yeah will lady rainicorn be there?" he asked happily.

Finn nodded "hell yeah, Slime princess, Flame princess, peppermint butler you name it!" he cried hyperactively.

Fionna looked very excited "Oh wow a lumpy space party I'm so excited!" she cried twirling about the room.

Marshall Lee looked pleased too "Sweet a lumpy Space party I can bring my jamming axe guitar!" he said playing air guitar.

_**At party**_

Finn, Jake, BMO, Marshall Lee and Fionna appeared in the flower garden where Lumpy space princess greeted them in a sarcastic show off manner.

"Like Jake, Finn you made it welcome to my like awesome party!" she said folding her arms in a show-off manner.

Fionna came forward with a present "Um happy birthday LSP we brought you a gift" she said smiling nervously.

Marshall Lee sighed crossly "I only came for the food in truth I think she's just spoiling herself more".

Cake bit Marshall to shut him up and Fionna mouthed thank you and handed Lumpy Space Princess her gift kindly still humiliated by her boyfriend's behaviour.

When Lumpy Space Princess opened it she was overwhelmed "Like oh my glob it's so lumping beautiful you guys!" she cried close to tears.

Inside the box was a gold tiara, a white lace ball gown with a blue gem in the chest and on either side of the sleeves inside 2 blue bows.

Jake looked amazed "Wow Fionna how did you pull that off I mean to even fit her size?" he asked curiously.

Lady Rainicorn appeared "Me and Fionna took her to get measured in secret by telling her it was a weight loss programme" (In Korean).

Jake looked amazed "Whoa Lady you are so caring I love you!" he said hugging Lady Rainicorn's face and stroking it.

Suddenly Fionna felt something slimy pat her leg making her jump and she turned to see Slime Princess smiling at her "Oh Slime princess hello this is the first time we've met I'm Finn's sister Fionna" she said kindly.

"EEEWWWW Gross" Marshall Lee said hiding behind Fionna who stamped on his foot crossly.

Jake looked confused "Then how do you know her name?" he asked curiously.

Cake sighed "Jake she's made of slime honey, it AINT hard to figure out!" she sighed crossly.

Suddenly wild berry princess came running up to them happily "Hi Finn, Jake, Fionna and Cake would you like some wild berry juice?" she asked sweetly batting her eyelashes.

"Hey what the glob it's my birthday give me some!" Lumpy Space Princess snapped.

They nodded and wild berry princess squeezed some red juice into some glasses and handed it out to the six friends and they gulped it down.

"Oh Man this juice is like radical Wild Berry Princess" Finn cried happily gulping down the delicious fruity goodness.

Wild Berry Princess blushed "Why thank you Finn it means a lot coming from you" she said shyly.

Jake said nothing and just shared his with Lady Rainicorn and kissed her saying it was as sweet as her love which made Finn sigh exasperatedly.

"Oh wow Bro this juice tastes so sweet it's great!" Fionna cried happily.

Cake purred happily "Oh baby this juice is tastier than Jakes giant sandwiches" she cried.

Marshall Lee sighed happily "Oh wow this red stuff is awesome I gotta visit her more often to feed" he said rubbing his tummy.

Lumpy Space Princess hit her face in shock "Oh my glob! This juice is the best lumping juice I ever tasted!" she cried and begged for more to which Wild berry agreed but only this once.

Marshall Lee put his arm around Fionna's waist making her blush "Hey Marshall let go you pervert!" she yelled crossly.

Marshall shook his head smiling "No way babe I'm too happy right now" he answered hugging her tightly.

Prince Gumball looked unhappy as he had a crush on Fionna "Oh Fionna why won't you look at me I could make you my queen!" he thought sadly.


	3. C3: Sleepover party disaster

Finn, Jake, Cake and Fionna were looking out their Pajama's and snack for the sleepover party they were hosting tonight. They were providing snacks and taking care of beds but had asked the guests to bring entertainment and their own choice of snacks plus music.

Jake began prepping a table with Chips and Dips plus some speakers that BMO would be attached to so music would be played "This party is gonna rock man" he said taking a sneaky mouthful of chips.

Finn was making beds downstairs and in Finn and Jake's room plus the spare room "Totally dude we have never had a bad party yet! This is gonna be totally awesome" he said cheerfully.

Fionna was setting up the Karaoke machine with Marshall Lee while Cake took care of the Disco ball and Laser pens. She and Marshall Lee kept flirting every now and again making Finn feel jealous of her.

_**Later**_

Lady Rainicorn was the first to arrive with Princess Bubblegum who had brought along Chocó berry, Mr. Cupcake, Cinnamon bun and Peppermint butler as guests. As food they had brought large amounts of candy while Lady Rainicorn had brought cookies and sandwiches.

Princess Bubblegum placed a mixed basket of candy on the table then turned to Finn "I am most grateful for you inviting me to your party Finn" she said politely.

Finn blushed "That's ok PB I'm glad you could make it" he said bashfully.

Lady Rainicorn kissed Jake on the cheek and hugged him then said placed the food she brought on the table.

Jake took a deep breath of the cookies warm smell "Ooo fresh out of the oven you're so good at cooking lady".

Lady Rainicorn chuckled bashfully "Well I knew they were your favourite Jake" (In Korean).

Jake turned red and chuckled bashfully his tail wagging happily.

Marceline was the second to arrive wearing her blue dress and striped stocking with heels she also brought her guitar and some red punch for beverages "Hey Finn thanks for inviting me this party is SO gonna rock" she said cheerfully.

Marshall Lee approached Marceline "neat nice guitar now we can have a duo concert" he said producing his own guitar.

Suddenly LSP arrived with wieners and popcorn "What the Glob Finn why did you tell me the wrong time?!" she said crossly.

Finn sighed exhaustedly "Nice to see you too LSP".

BMO tugged on Fionna's sock "Can you plug me in so I can play music" he asked.

Fionna smiled "Sure" she said and plugged BMO in at his jack plug and the music began so everyone started dancing.

Marceline and Marshall Lee began rocking on their guitars and even Princess Bubblegum couldn't help dancing to the music along with her fellow candy people.

_**Later**_

Fionna stood at the mike of the Karaoke machine "Ok guys it's time to party and who will do us the honour of singing first?" she called out excitedly.

LSP pushed to the front "Oh…Oh…Oh…Me I have an awesome song that is totally lumping awesome and will blow you all the lump away!" she said proudly. Fionna shrugged and handed the mike to LSP who cleared her throat and then sang:

"_These Lumps"_

"_I know you wanna slump upon these Lumps but you can't cos you're a chump"_

"_A Chuuuummmp!"_

There was a long silence and someone coughed and some people gave a quiet clap to be polite but it wasn't that great.

LSP tightened her fists and clenched her teeth crossly "FINE I'm BETTER THAN YOU ALL ANYWAY" she yelled crossly and went over to the food table and began to gorge on candy.

Next up was Marshall Lee and Fionna singing bad little boy which got everyone excited by the intense romantic mood which annoyed LSP and made her even sulkier.

After Karaoke there was more dancing but those who wanted a rest just began to talk about stuff on comfy sofas and chairs etc.

Bubblegum and Gumball tried to approach Fionna and Finn but Marshall Lee and Marceline got to them first and started talking to them greatly agitating them so they began to sulk in the corner while stuffing their faces full of candy.

Peppermint butler had some punch and talked to Cinnamon bun about the party and royal matters. Chocó berry and Mr. Cupcake danced together like crazy with laser pens like most of the others.

After the party finally reached its climax everyone decided to crash wherever they fell and most ended up in the most awkward of places.

Finn and Jake slept in their own room with BMO enjoying their comfy beds, Fionna and Marshall Lee shared a bed in their own private room like they usually did.

Lady Rainicorn slept on the roof as she didn't mind sleeping outside as she was used to it and it was the middle of summer so it was rather warm outside.

Chocó berry, Mr. Cupcake, Cinnamon Bun and peppermint butler slept in homemade hammocks in the kitchen hung on the rafters.

Princess Bubblegum and Prince Gumball slept downstairs in sleeping bags and although they disliked sleeping on the hard ground they did not argue.

Marceline slept on the sofa and was muttering about red things and how she was gonna sink her teeth into them which annoyed bubblegum and awful lot while she was trying to sleep.

LSP had decided to sleep in the bedding cupboard since it was warm and cozy in there and some of the sheets smelled like Finn whom she had a crush on.

Finn yawned "You know what man" he said sleepily turning to Jake.

Jake yawned sleepily scratching his tummy "No what?" he said sleepily.

Finn smiled sleepily closing his eyes "Tonight was totally awesome" he said falling asleep.

Jake smiled "You got that right" he said falling asleep.


End file.
